1. Field of Invention
The present invention relates to a chemical-mechanical polishing (CMP) station. More particularly, the present invention relates to a chemical-mechanical polishing station having a device for monitoring the progress of a wafer polishing operation and facilitating the determination of a polishing end-point.
2. Description of Related Art
Semiconductor fabrication has reached the deep submicron stage. In the deep submicron stage, the feature size and the depth of focus of photolithographic equipment are reduced, and the number of multi-level metal interconnect layers is increased. Consequently, how to maintain a high degree of surface planarity for a wafer becomes a major topic of investigation.
Before the deep submicron era of semiconductor production, spin-on-glass used to be the principle method of planarizing a silicon wafer. However, the method can obtain moderate planarity only in local areas on the wafer surface. Without a global planarization of the wafer surface, quality of development after photographic exposure is poor and the etching end-point is difficult to determine. Hence, yield of wafers is low.
Chemical-mechanical polishing is now the principle means of globally planarizing a silicon wafer, especially in the process of forming deep submicron circuits that have a feature size smaller than 0.18 .mu.m. In addition, copper has gradually replaced aluminum as the material for forming conductive lines inside a wafer in a so-called damascene process. Since copper is difficult to remove with a common etchant, a chemical-mechanical polishing operation must be used instead.
FIG. 1 is a sketch of the components of a conventional chemical-mechanical polishing station for polishing wafer. As shown in FIG. 1, a wafer 18 is held firmly inside the retaining ring 16a of a polishing head 16. The polishing head 16 provides the rotation necessary for polishing as well as the means to lower the wafer 18 onto a polishing table having a polishing pad 10 that rotates in a direction opposite to that of polishing head 16. A slurry supplier 12 is also mounted above the polishing pad 10 to provide slurry 14 for carrying out the polishing action. The slurry 14 contains some polishing agents; among them are included particles of metallic oxide that provide abrasive action necessary for polishing the wafer 18. To prevent over-polishing of the wafer 18, the polishing head 16 is lifted from the polishing pad 10 after a predetermined time interval.
However, due to the unrepeatable amounts of the ingredients within the slurry and conditions of the polishing pad 10 as well as the unpredictability of the wafer surface, appropriate parameter settings are difficult to decide beforehand. Consequently, either too much or too little metal atop a dielectric layer is removed in a damascene process. When too much metal is removed, it causes metal pattern dishing and erosion during over-polishing, and the electrical properties suffer. When too much metal is removed on the wafer surface, it causes a metal bridge effect, and the wafer yield suffers.